U.S. patent application number 14/163575 was filed with the patent office on 2014-05-22 for fixed multiple access wireless communication.
This patent application is currently assigned to COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION. The applicant listed for this patent is COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION. Invention is credited to Zhuo Chen, Iain Bruce Collings, Douglas Brian Hayman, Joseph Abraham Pathikulangara, Hajime Suzuki.
Application Number | 20140140320 14/163575 |
Document ID | / |
Family ID | 41796627 |
Filed Date | 2014-05-22 |
United States Patent
Application |
20140140320 |
Kind Code |
A1 |
Suzuki; Hajime ; et
al. |
May 22, 2014 |
FIXED MULTIPLE ACCESS WIRELESS COMMUNICATION
Abstract
Disclosed is a user terminal for wireless communication with a
remote access point, the user terminal comprising: a mapping module
adapted to map one or more input data bits to an uplink symbol; a
delay module adapted to apply a delay to the uplink symbol; a
transmit module adapted to modulate the delayed symbol into a
frequency channel; and a directional antenna oriented along a
dominant path to the access point, the antenna being adapted to
transmit the modulated symbol to the access point, wherein the
delay is chosen such that the transmitted symbol arrives at the
access point simultaneously with a further symbol modulated into
the frequency channel and transmitted by a further user
terminal.
Inventors: |
Suzuki; Hajime; (Epping,
AU) ; Hayman; Douglas Brian; (Macquarie Park, AU)
; Pathikulangara; Joseph Abraham; (Epping, AU) ;
Collings; Iain Bruce; (Roseville Chase, AU) ; Chen;
Zhuo; (Chatswood, AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH
ORGANISATION |
Campbell |
|
AU |
|
|
Assignee: |
COMMONWEALTH SCIENTIFIC AND
INDUSTRIAL RESEARCH ORGANISATION
Campbell
AU
|
Family ID: |
41796627 |
Appl. No.: |
14/163575 |
Filed: |
January 24, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12746911 |
Aug 27, 2010 |
8675512 |
|
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14163575 |
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Current U.S.
Class: |
370/330 |
Current CPC
Class: |
H04L 5/0005 20130101;
H04B 7/08 20130101; H04W 56/0045 20130101; H04B 7/0617 20130101;
H04W 56/006 20130101; H04B 7/0408 20130101 |
Class at
Publication: |
370/330 |
International
Class: |
H04L 5/00 20060101
H04L005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2008 |
AU |
2008904556 |
Aug 10, 2009 |
AU |
PCT/AU2009/001022 |
Claims
1-4. (canceled)
5. A system for wireless communication comprising: an access point
adapted to wirelessly communicate data symbols in at least one of:
two or more different frequency channels, and two or more different
time slots of a frequency channel; and a plurality of user
terminals, each said user terminal being adapted to communicate
said data symbols in one said time slot of one said frequency
channel, wherein each user terminal with the same azimuthal
ordering module or the number of said different frequency channels
or time slots is adapted to communicate said data symbols in the
same said frequency channel and in the same said time slot.
6. (canceled)
7. The system of claim 5, wherein said access point further
comprises a precoding module adapted to precode said data symbols
before transmitting, said precoding being dependent on information
about at least one said frequency channel between said access point
and said user terminals.
8. The system of claim 5, wherein said access point further
comprises a plurality of vertically polarized antennas disposed in
an array, wherein two adjacent said antennas in said array are
dislocated vertically relative to one another.
9. An access point configured to wirelessly communicate data
symbols with user terminals in at least one of two or more
different frequency channels and two or more different time slots
of a frequency channel, said access point comprising a precoding
module configured to precode said data symbols before transmitting,
said precoding being dependent on information about at least one
said frequency channel between said access point and said user
terminals.
10. The access point of claim 13, further comprising a plurality of
vertically polarized antennas disposed in an array, wherein two
adjacent said antennas in said array are vertically dislocated
relative to one another.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to wireless
communication and, in particular, to wireless communication between
sparsely distributed fixed user stations and a fixed access
point.
BACKGROUND
[0002] Providing an inexpensive high-capacity bidirectional data
link to user terminals in remote areas poses many challenges.
Because user terminals in remote areas are typically distributed
sparsely over a large geographic area (e.g. tens of terminals over
hundreds of square kilometres), the cost of deploying a wired
network is prohibitive. Wireless communication networks, with a
point-to-multipoint topology comprising a network hub or access
point with which multiple user terminals communicate independently
and bidirectionally, are a more promising technology to deploy.
[0003] In digital broadcasting a video stream of 20 MBits/sec can
be delivered from an access point to any number of user terminals
over a radius of tens of kilometres within a 7 MHz bandwidth in the
VHF frequency band. However, in a broadcasting application the data
is unidirectional and common to all user terminals, so the required
capacity to service all users is independent of the number of user
terminals.
[0004] Candidate wireless technologies for independent
bidirectional data transmission such as WiMAX (IEEE 802.16), which
typically operates at a carrier frequency above 2 GHz, suffer from
two related problems: [0005] 1. Inadequate coverage. The distance
between an access point and a user terminal is limited to less than
10 kilometres at a carrier frequency above 2 GHz using an access
point antenna height of less than 30 m in a point-to-multipoint
topology. [0006] 2. Inadequate capacity. Current WiMAX technology
typically provides a spectral efficiency of 2 to 5 bits/sec/Hz
(i.e. 20 to 50 MBits/sec per 10 MHz frequency channel). This
capacity needs to be shared among, potentially, thousands of users.
To provide simultaneous access at data rates of 1 to 20 MBits/s to
this number of users from a single access point requires a
prohibitively large bandwidth at the carrier frequency.
[0007] There is a tradeoff between these two problems in that
capacity can be sacrificed for coverage, or vice versa, by
decreasing or increasing the carrier frequency respectively. A
possible way out of the tradeoff is to increase the transmit power
from the access point and the user terminals. This however
increases the cost of the system.
[0008] A satisfactory compromise providing acceptable bidirectional
data rates to all users in a sufficiently wide coverage area at low
enough power levels to yield acceptable cost is yet to be found
with WiMAX or other conventional technologies.
SUMMARY
[0009] It is an object of the present invention to substantially
overcome, or at least ameliorate, one or more disadvantages of
existing arrangements.
[0010] Disclosed are arrangements which seek to address the above
problems, for example, a wireless communication system in which
multiple user terminals are accurately synchronised in time and
frequency to allow the parallel uplink data streams from the user
terminals to be effectively separated at the access point. Because
the system relies predominantly on line-of-sight transmission, the
user terminal antennas are directional, saving power on the
uplink.
[0011] According to a first aspect of the present disclosure, there
is provided a user terminal for wireless communication with a
remote access point, the user terminal comprising a mapping module
adapted to map one or more input data bits to an uplink symbol; a
delay module adapted to apply a delay to the uplink symbol; a
transmit module adapted to modulate the delayed symbol into a
frequency channel; and a directional antenna oriented along a
dominant path to the access point, the antenna being adapted to
transmit the modulated symbol to the access point, wherein the
delay is chosen such that the transmitted symbol arrives at the
access point simultaneously with a further symbol modulated into
the frequency channel and transmitted by a further user
terminal.
[0012] According to a second aspect of the present disclosure,
there is provided a system for wireless communication comprising an
access point adapted to wirelessly communicate data symbols in at
least one of two or more different frequency channels, and two or
more different time slots of a frequency channel; and a plurality
of user terminals, each user terminal being adapted to communicate
the data symbols in one time slot of one frequency channel, wherein
each user terminal with the same azimuthal ordering modulo the
number of different frequency channels or time slots is adapted to
communicate the data symbols in the same frequency channel and in
the same time slot.
[0013] Other aspects are also disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] One or more embodiments of the present invention will now be
described with reference to the drawings, in which:
[0015] FIG. 1a is an illustration of a wireless communication
system in which the embodiments of the present invention may be
practised;
[0016] FIG. 1b illustrates, in exaggerated scale, the access point
and one of the user terminals of FIG. 1a in more detail;
[0017] FIG. 2 is a block diagram of the signal processing system
for the downlink at the access point of FIG. 1b;
[0018] FIG. 3 is a block diagram of the signal processing system
for the downlink at the user terminal of FIG. 1b;
[0019] FIG. 4 is a block diagram of the signal processing system
for the uplink at the user terminal of FIG. 1b;
[0020] FIG. 5 is a block diagram of the signal processing system
for the uplink at the access point of FIG. 1b;
[0021] FIG. 6 illustrates a network with 12 user terminals sparsely
distributed around an access point within an annular region bounded
by two concentric circles;
[0022] FIG. 7 shows an example of grouping user terminals to
provide differentiated data rates;
[0023] FIG. 8 shows an example of an arrangement of access point
antennas to reduce mutual coupling; and
[0024] FIG. 9 illustrates the operation of the Delay module in the
user terminal uplink system of FIG. 4.
DETAILED DESCRIPTION
[0025] Where reference is made in any one or more of the
accompanying drawings to steps and/or features, which have the same
reference numerals, those steps and/or features have for the
purposes of this description the same function(s) or operation(s),
unless the contrary intention appears.
[0026] FIG. 1a is an illustration of a wireless communication
system 100 in which the embodiments of the invention may be
practised. The system 100 includes an access point 105 in
bidirectional wireless communication in a single time slot of a
single frequency channel with M user terminals, e.g. 115, at fixed
locations sparsely distributed within a circular area 125, with a
typical radius of tens of kilometres. In the illustration M=8, but
any value of M is possible up to and including the number N of
access point antennas. The access point is 105 typically connected
to another network, for example the public-switched telephone
network.
[0027] FIG. 1b illustrates, in exaggerated scale, the access point
105 and one of the user terminals 115 of the system 100 in more
detail. The access point 105 includes an array 110 of N vertically
polarised antennas uniformly arranged in a horizontal circle,
elevated from the ground by mounting on a tower. The antenna array
110 is used for both transmitting and receiving data. The
bidirectional communication is performed in a time division
duplexing (TDD) manner. The circular array 110 is just one example
of an arrangement of antennas at the access point 105; other
possible arrangements are linear, square, and arc. The spacing of
the antenna array 110 need not be uniform. The performance of the
system 100, as described below, improves as the antenna spacing
increases as a ratio of the carrier wavelength, but clearly there
are practical limits on the spacing. The access point 105 antennas,
illustrated as half-wave dipoles, may be of any omni-directional
design. Directional antennas can also be used at the access point
105, provided that the number of simultaneous user terminals
operating in the same frequency channel within any particular
region does not exceed the number of access point directional
antennas covering the region. In this case, each region may be
treated independently as serviced by a single instance of the
system 100.
[0028] Also shown in FIG. 1b is a user terminal 115, illustrated as
a house with a directional antenna (illustrated as a Yagi antenna)
120 mounted thereon, used for both transmitting and receiving data.
In the remote-area environment for which the system 100 is
designed, the line-of-sight signal path 130 between the user
terminal antenna 120 and the access point array 110 is usually the
dominant path, with the only other signal path of significance
being a ground reflection path 140. The main beam of the antenna
120 is therefore oriented along the direction of the access point
105. Alternatively, if it is known that at a particular user
terminal location, the dominant path to the access point is not
line-of-sight (e.g. reflection from a mountain), then the main beam
of the user terminal antenna 120 can be oriented along the
non-line-of-sight dominant path.
[0029] FIG. 2 is a block diagram of the signal processing system
200 for the downlink (i.e. transmission from the access point 105
to the user terminal 115) at the access point 105 of the system
100. Binary downlink data (DD) intended for the m-th user terminal
(UT) 115 (m=1, 2, . . . , M), typically obtained from the network
to which the access point 105 is connected, is routed through the
UT.sub.m DD module 210-m. The downlink data is optionally coded by
a forward error correction (FEC) encoder (not shown) at the cost of
some data redundancy, i.e. a reduced data rate. The binary downlink
data is then mapped onto a multi-level quadrature amplitude
modulation (M-QAM) or a multi-level phase shift keying (M-PSK)
symbol constellation, by the Map module 220-m to produce downlink
data symbols s.sub.D,m to be transmitted to the m-th user terminal.
The M-QAM or M-PSK transmitted symbols are allocated by the Map
module 220-m to bit groupings with q bits per symbol.
[0030] Channel information, obtained from a channel estimation
module 235, is used by a zero-forcing precoder (ZFP) module 230 to
perform zero-forcing precoding on the downlink data symbols
s.sub.D,m as described below. The downlink channel information is
obtained from the uplink channel information using the principle of
reciprocity as described below. The uplink channel information is
estimated by sending training signals from the user terminals to
the access point. The training signals are known both to the user
terminals and the access point prior to the transmission. The
training signal from one user terminal is orthogonal to the
training signals from all other user terminals. For example, a
training signal is sent from only one user terminal at one time so
that the information for the uplink channels from the user terminal
antenna to N access point antennas can be estimated without
interference from the transmissions of the other users. In other
embodiments, the training signals from different users can be made
orthogonal in frequency or in code.
[0031] The resulting precoded symbols are scaled by a common
factor, and the scaled precoded symbols x.sub.D,n (n=1, 2, . . . ,
N) are modulated onto a common carrier in the frequency channel by
an access point transmit (AP.sub.n Tx) module 240-n and transmitted
via a corresponding transmit antenna 250-n which is part of the
access point array 110.
[0032] FIG. 3 is a block diagram of the signal processing system
300 for the downlink at the m-th user terminal 115 (m=1, 2, . . . ,
M) of the system 100. The m-th user terminal receive (UT.sub.m Rx)
module 310 receives and demodulates symbols r.sub.D,m from the
antenna 305. Each received symbol r.sub.D,m is scaled by a scaling
factor .beta., defined below, at the scaling module 320 to produce
scaled symbols z.sub.D,m. The detection of downlink symbols
s.sub.D,m from the scaled received symbols z.sub.D,m is performed
by the DET module 330 as described below. The De-map module 340
performs de-mapping of detected data symbols s.sub.D,m to binary
data according to the symbol constellation used by the map module
220-m. The binary downlink data is passed to the data sink
(UT.sub.m DS) module 350. In the case of FEC coded transmission,
the DET module 330 includes a soft decision estimator, the De-map
module 340 includes a bit value probability estimator, and the
UT.sub.m DS module 350 includes a FEC decoder (not shown).
[0033] The zero-forcing precoding allows the downlink portion of
the system 100 to function like an SDMA
(space-division-multiple-access) system whereby symbols sharing a
single timeslot and a single frequency are transmitted to be
received by corresponding user terminals at different
locations.
[0034] FIG. 4 is a block diagram of the signal processing system
400 for the uplink (transmission from the user terminal 115 to the
access point 105) at the m-th user terminal 115 (m=1, 2, . . . , M)
of the system 100. The bidirectional communication is performed in
a TDD manner. Input binary uplink data (UD) from the m-th user
terminal is generated in the UT.sub.m UD module 410. The uplink
data from the UT.sub.m UD module 410 is optionally coded by an FEC
encoder (not shown). The binary uplink data from the UT.sub.m UD
module 410 is then mapped by the Map module 420 onto a M-QAM or
M-PSK symbol constellation to produce uplink data symbols
s.sub.U,m. The time information from a Global Positioning System
(GPS) receiver 440 is used by the Delay module 430 to synchronise
the transmission of symbols from the user terminal 115 with the
other user terminals. A commercially available GPS module that is
capable of providing a timing accuracy of less than 15 ns and a
frequency accuracy of less than 30 parts per billion (ppb) can be
used for this purpose. The accurate time information available from
the public data of the GPS allows user terminals to synchronise
their transmission to within 15 ns, which is effectively
simultaneous for equalisation purposes at the frequency channels in
use. The function of the delay module 430 is to ensure that the
transmitted symbols from all the user terminals are simultaneously
(to symbol precision) received at the access point 105 regardless
of the location of the user terminals. The propagation delay due to
the distance from the access point 105 is determined, for example,
from the location of the user terminal 115 given by the GPS
receiver 440, or from the time of arrival computed with reference
to the time information provided by the GPS receiver 440 of an
accurate time signal sent from the access point 105 to the user
terminal 115. The propagation delay is taken into account by the
Delay module 430 to compute the delay that is applied by the Delay
module 430.
[0035] The operation of the delay module 430 is described with
reference to FIG. 9. Three sequences 910, 915, and 920 of uplink
data symbols s.sub.U,m(i) from three user terminals (m=1, 2, 3) are
shown against the time axis 905, starting from the instant 925
representing the beginning of the user terminals' time frame with
reference to their respective GPS time information. If no delay is
applied by Delay module 430, the symbol sequences 910, 915, and 920
become receive symbol sequences r.sub.U,n(i) 935, 940, and 945 on
arrival at the access point antennas (n=1, 2, 3). Because the user
terminals are at different distances from the access point, each
received symbol sequence is delayed by a respective propagation
delay 932, 937, or 942, and are therefore no longer synchronised
with each other. If however each Delay module 430 applies a delay
957, 962, or 967 that is complementary to the corresponding
propagation delay 932, 937, or 942 to form a delayed uplink symbol
sequence x.sub.U,m(i) 955, 960, or 965 respectively, the
combination of the applied delays and the propagation delays
results in received symbol sequences r.sub.U,n(i) 975, 980, and 985
that are synchronised at the access point at the instant 990.
[0036] A conventional SDMA access point needs to perform symbol
synchronisation (which determines the beginning of each symbol) and
carrier offset correction (which determines the difference in
frequency between the frequency reference used in a user terminal
and the frequency reference used at the access point) for each user
terminal. By taking into account the propagation delay at the
transmission from the user terminals, and thereby synchronising the
reception at the access point to symbol precision, the access point
needs to perform symbol synchronisation only once for all user
terminals. Similarly, by using the same frequency reference
obtained from the GPS signal at every user terminal, the access
point needs to perform carrier offset correction only once for all
user terminals. If the access point also uses the same frequency
reference obtained from the GPS signal, then no carrier offset
correction is required. The effects of Doppler shift are small due
to the existence of dominant line-of-sight path and the fixed
access point and user terminals. This greatly simplifies the signal
processing required to detect symbols from each user terminal
received at the same time slot in the same frequency channel.
Notably, conventional MIMO signal processing techniques, such as
V-BLAST, can then be used within the system 100.
[0037] The delay module 430 also scales each uplink data symbol
s.sub.U,m as described below. The scaled, delayed uplink symbol
x.sub.U,m is modulated onto a common carrier in the frequency
channel by the user terminal transmit (UT.sub.m Tx) module 450 and
transmitted by the antenna 460. The directional nature of the
antenna 460 (typically with an antenna gain of 10 to 20 dBi)
enables the transmitted power for the uplink to be much lower than
would be required if the antenna 460 were omni-directional to
provide the same SNR at the access point 105.
[0038] The carrier frequency reference for the UT.sub.m Tx module
450 is given by the GPS receiver 440, so that the frequency
reference of each user terminal 115 is synchronised with the access
point 105 to an accuracy of, for example, 30 parts per billion. The
time and frequency synchronisation of the multiple user terminals,
together with the multiple antennas at the access point 105, gives
the system 100 the characteristics of a multiple-input
multiple-output (MIMO) system, which is normally employed to
increase the capacity of a link between two terminals in conditions
of severe multipath propagation. By contrast with the system herein
disclosed, conventional MIMO systems utilise omni-directional
antennas at both terminals to maximise the diversity order of the
multipath channel.
[0039] FIG. 5 is a block diagram of the signal processing system
500 for the uplink at the access point 105 of the system 100. The
n-th (n=1, 2, . . . , N) access point receive (AP.sub.n Rx) module
520-n receives a signal from a corresponding antenna 510-n and
demodulates symbols r.sub.U,n from the received signal.
Zero-forcing equalisation is performed by the module 530 as
described below to produce equalised symbols z.sub.U,m (m=1, 2, . .
. , M). The detection of transmitted symbols s.sub.U,m from the
equalised symbols z.sub.U,m is performed by the DET module 540-m as
described below. The De-map module 550-m performs de-mapping of
detected data symbols s.sub.U,m to binary uplink data, which is
passed onto the m-th user terminal uplink data sink (UT.sub.m US)
module 560-m. In the case of FEC coded transmission, the DET module
540-m includes soft decision estimation, the De-map module 550-m
includes bit value probability estimation, and the UT.sub.m US
module 560-m includes a FEC decoder (not shown).
[0040] The downlink channel, through which downlink transmit
symbols x.sub.an from the n-th access point transmitter 240-n
become received symbols r.sub.am at the m-th user terminal receive
module 310, is modelled as a matrix multiplication:
[ r D , 1 r D , 2 r D , M ] = [ g D , 1 , 1 g D , 1 , 2 g D , 1 , N
g D , 2 , 1 g D , 2 , 2 g D , 2 , N g D , M , 1 g D , M , 2 g D , M
, N ] [ x D , 1 x D , 2 x D , N ] + [ n D , 1 n D , 2 n D , M ] ( 1
) ##EQU00001##
[0041] where g.sub.D,m,n is the complex-valued (m, n)-th element of
the downlink channel matrix G.sub.D (M rows by N columns), and
n.sub.D,m is the additive noise at the m-th user terminal receive
module 310.
[0042] Equation (1) may be rewritten as
r.sub.D=G.sub.D.times.x.sub.D+n.sub.D (2)
[0043] Define a pseudo-inverse, W.sub.D, of G.sub.D as follows:
W.sub.D=(G.sub.D.sup.HG.sub.D).sup.-1G.sub.D.sup.H (3)
[0044] where H indicates the Hermitian (complex conjugate
transpose) of a matrix.
[0045] W.sub.D is a N.times.M matrix enumerated as
W D = [ w D , 1 , 1 w D , 1 , 2 w D , 1 , M w D , 2 , 1 w D , 2 , 2
w D , 2 , M w D , N , 1 w D , N , 2 w D , N , M ] ( 4 )
##EQU00002##
[0046] that satisfies
W.sub.DG.sub.D=I.sub.N
[0047] The zero-forcing pre-coding carried out at the access point
105 by the module 230 is defined as follows:
x D = NP D W D 2 W d s D where ( 5 ) s D = [ s D , 1 s D , 2 s D ,
M ] ( 6 ) ##EQU00003##
[0048] is the vector of user terminal downlink symbols, P.sub.D is
a time-averaged transmitting power from an access point transmit
antenna 250-n, and
W D 2 = n = 1 N m = 1 M w D , n , m 2 ( 7 ) ##EQU00004##
[0049] The scaling by {square root over
(NP.sub.D/.parallel.W.sub.D.parallel..sup.2)} if makes sure that
the total transmitting power from the access point transmitters
240-n is constrained to NP.sub.D.
[0050] Substituting (5) into (2) gives
r D = NP D W D 2 G D W D s D + n D = NP D W D 2 s D + n D ( 8 )
##EQU00005##
[0051] The scaling factor .beta. used by the scaling module 320 at
each user terminal before detection of downlink symbols is defined
as {square root over
(.parallel.W.sub.D.parallel..sup.2/(NP.sub.D))}, so that:
z D = W D 2 NP D r D = s D + W D 2 NP D n D ( 9 ) ##EQU00006##
[0052] or, at the user terminal m,
z D , m = s D , m + W D 2 NP D n D , m ( 10 ) ##EQU00007##
[0053] The value of .beta. is provided at each user terminal 115
prior to the reception of data symbols. This can be achieved, for
example, by sending a known reference signal from the access point
105. While an accurate value of .beta. at the user terminal 115
improves the accuracy of the de-mapping, the scaling factor .beta.
can also be estimated at the user terminal 115 from, for example,
the variance of the received symbols, provided that the variance of
the noise component is smaller than that of the signal
component.
[0054] The detection of transmitted symbols is performed by the DET
module 330 as a "hard decision":
s ^ D , m = arg min s i .di-elect cons. Q z D , m - s i ( 11 )
##EQU00008##
[0055] where s.sub.i, i=1, 2, . . . , 2.sup.q, is the i-th symbol
in the chosen M-QAM or M-PSK constellation Q.
[0056] Like the downlink channel, the uplink channel is modelled as
a matrix multiplication:
[ r U , 1 r U , 2 r U , N ] = [ g U , 1 , 1 g U , 1 , 2 g U , 1 , N
g U , 2 , 1 g U , 2 , 2 g U , 2 , N g U , N , 1 g U , N , 2 g U , N
, M ] [ x U , 1 x U , 2 x U , M ] + [ n U , 1 n U , 2 n U , N ] (
12 ) ##EQU00009##
[0057] where r.sub.U,n and n.sub.U,n are the received uplink symbol
and the noise respectively at the n-th access point receive module
520-n, x.sub.U,m is the transmitted uplink symbol from the m-th
user terminal transmit module 450, and g.sub.U,n,m is the
complex-valued uplink channel coefficient between the m-th user
terminal transmit module 450 and the n-th access point receive
module 520-n.
[0058] Equation (12) may be rewritten in matrix form as
r.sub.U=G.sub.Ux.sub.U+n.sub.U (13)
[0059] where G.sub.U is the N by M matrix whose (n, m)-th entry is
g.sub.U,n,m.
[0060] Since the same frequency channel is used for the downlink
and the uplink, the reciprocity principle states that
g.sub.D,m,n=g.sub.U,n,m, or
G.sub.D=G.sub.U.sup.T (14)
[0061] where the superscript T indicates the transpose of a
matrix.
[0062] Write
s U = [ s U , 1 s U , 2 s U , M ] ( 15 ) ##EQU00010##
[0063] where s.sub.U,m is a M-QAM or M-PSK uplink data symbol from
the user terminal uplink mapping module 420. Then the scaling at
the delay module 430 of the user terminal 115 is
x.sub.U,m= {square root over (P.sub.U)}s.sub.U,m (16)
[0064] where P.sub.U is the time-averaged transmit power of each
user terminal antenna 460. (The transmit power from each user
terminal transmit antenna 120 is the same.) Like the scaling in the
module 320, the scaling by {square root over (P.sub.U)} makes sure
that the transmitting power from the user terminal transmit antenna
460 is constrained to P.sub.U.
[0065] Define a pseudo inverse, W.sub.U, of G.sub.U as
W.sub.U=(G.sub.U.sup.HG.sub.U).sup.-1G.sub.U.sup.H (17)
[0066] W.sub.U is a M.times.N matrix that satisfies
W.sub.UG.sub.U=I.sub.M (18)
[0067] Note that, because of equations (3) and (14),
W.sub.U.sup.T=W.sub.D (19)
[0068] The zero-forcing equalisation performed by the module 530 of
the access point 105 is defined using the uplink channel
pseudo-inverse matrix W.sub.U as follows:
z U = 1 P U W U r U = 1 P U W U G U x U + 1 P U W U n U = s U + 1 P
U W U n U or ( 20 ) z U , m = s U , m + 1 P U n = 1 N w U , m , n n
U , n ( 21 ) ##EQU00011##
[0069] The values of W.sub.U/ {square root over (P.sub.U)} are
provided to the access point uplink equaliser 530 prior to the
reception of data symbols. This can be achieved, for example, by
sending known reference signals from each user terminal to the
access point 105.
[0070] The detection of uplink data symbols from the m-th user
terminal is performed by the Det module 540-m as a "hard
decision":
s ^ U , m = arg min s i .di-elect cons. Q z U , m - s l ( 22 )
##EQU00012##
[0071] where i=1, 2, . . . , 2.sup.q, is the i-th symbol in the
chosen M-QAM or M-PSK constellation Q.
[0072] Alternatives to zero-forcing pre-coding such as regularised
inverse pre-coding and vector perturbation can be used at the
module 230 of the access point 105. Similarly, for the uplink
transmission, conventional spatial multiplexing MIMO equalisation
techniques, such as list sphere detection and V-BLAST, can be used
as alternatives to zero-forcing equalisation at the module 530 of
the access point 105. Also, wideband modulation by conventional
techniques such as orthogonal frequency division multiplexing
(OFDM) is an alternative to single frequency carrier modulation at
the transmit modules 240-n and 450.
[0073] The capacity of each link in bits/s/Hz is directly
proportional to the value of M, subject to the requirement that
M.ltoreq.N, as long as the bit error rate (BER) is very small.
However, as M approaches N, the BER increases. Other factors that
adversely affect the BER are: [0074] Lower signal-to-noise ratio
(SNR) at the user terminal (downlink) and at the access point
(uplink); [0075] Denser symbol constellation (i.e. larger value of
q); [0076] Smaller separation of the antennas in the access point
antenna array 110; [0077] Greater distance between the access point
105 and the user terminals 115; [0078] Smaller angular separation
between the user terminals 115.
[0079] Using FEC encoding as described above, erroneous bits can be
corrected at the cost of redundancy, so a rise in BER translates to
a fall in error-free data rate.
[0080] A practical upper limit on M, given other system parameters
(e.g. maximum distance of 60 km, a carrier frequency of 600 MHz, a
16-point QAM symbol constellation, a spectral efficiency of 2M
bits/sec/Hz, an uncoded BER of 1%, and an SNR of up to 100 dB) is
roughly N/2, while the separation between antennas in the access
point array 110 should be at least half the wavelength of the
carrier.
[0081] To increase the number M of user terminals served by a
particular access point beyond the practical limit, additional
frequency channels may be allocated. For example, 10 frequency
channels can be utilised by one access point with 100 antennas to
serve 500 user terminals simultaneously, where each of 10 frequency
channels serves one group of 50 user terminals. Alternatively, the
capacity of one frequency channel may be shared between multiple
user groups by assigning the groups to different time slots. With
10 time slots, 500 user terminals can be served within one
frequency channel, where each user terminal obtains one tenth of
the original data rate. In these cases, assuming predominantly
line-of-sight propagation, there is a choice in how the 500
sparsely distributed user terminals are grouped into 10 groups.
[0082] FIG. 6 illustrates a network 600 with 12 user terminals,
e.g. 605, sparsely distributed around an access point 610 within an
annular region bounded by two concentric circles marking a minimum
and a maximum range. The user terminals are ordered from 1 to 12 by
azimuth from a reference direction 620, as shown in FIG. 6. As
mentioned above, transmitting independent data to two different
user terminals is more error-prone when the two user terminals are
located with a small angular (azimuthal) separation, for example
the user terminals 4 and 5. This effect can be minimised by
assigning user terminals with small azimuthal separation into
different groups. In the network 600, the 12 user terminals would
be grouped into two groups as follows: Group 1 consists of user
terminals {1, 3, 5, 7, 9, 11} and Group 2 consists of user
terminals {2, 4, 6, 8, 10, 12}. This grouping paradigm maximises
the minimum azimuthal separation between each pair of user
terminals that are adjacent in order of azimuth within a group.
[0083] More generally, the user terminal indices (after ordering
user terminals by azimuth) making up the k-th group out of K
groups, given a total number of user terminals KM, are
{k+(l-1)K,l=1,2, . . . ,M}.
[0084] i.e. Group k comprises those user terminals whose azimuthal
ordering index is equal to k modulo K. In this grouping paradigm,
the required SNR to achieve the same BER performance is the same
for both groups. In other words, the error-free data rates
achievable at the same SNR are the same for both groups.
[0085] In a system with both multiple time slots and multiple
frequency channels available, the grouping may be dynamic, because
not all user terminals wish to send or receive data at every time
slot. In this embodiment, for each time slot, the user terminals
wishing to send or receive data will be grouped by azimuth among
the frequency channels. This further increases the average
azimuthal separation between user terminals in the same group.
[0086] An alternative to grouping user terminals so as to provide
the same performance (BER vs SNR) among all user terminals is to
group user terminals to provide differentiated performance.
Grouping in this paradigm can be based on distance from the access
point, propagation path loss, or received power at the user
terminal.
[0087] FIG. 7 shows an example 700 of grouping user terminals to
provide differentiated data rates. In the first time slot (or the
first frequency channel), users 1 to 6, closer to the access point
710 than a threshold distance 720, are grouped together. In the
second time slot (or the second frequency channel), users 7 to 12,
further away from the access point 710 than the threshold distance
720, are grouped together. In this grouping paradigm, the required
SNR to achieve the same BER performance is smaller for the first
group than for the second group. In other words, the error-free
data rates achievable at the same SNR are higher for the first
group than for the second group.
[0088] With half-wavelength spacing of the antennas in the access
point array 110 as described above, the effect of mutual coupling
between antennas may significantly reduce the performance. The
effect of mutual coupling can be reduced if the access point
antennas are displaced vertically. FIG. 8 shows an example of such
an arrangement 800 of the access point antennas, e.g. 810, to
reduce mutual coupling. In the arrangement 800, adjacent vertically
polarised antenna elements 810 are still separated by half a
wavelength horizontally, but they are also displaced vertically by
their length, to reduce mutual coupling. The vertical displacement
is cyclical with a period of four antennas. The nearest
horizontally collocated antennas, 810 and 820, are separated by two
wavelengths, in which case the effect of mutual coupling is
insignificant. The spacing of horizontally collocated antennas can
be increased by increasing the period of the cyclical vertical
displacement.
[0089] Each module of FIGS. 2 to 5 is preferably implemented in
dedicated hardware such as one or more integrated circuits
performing the functions or sub-functions of the module as
described above. Such dedicated hardware may include graphic
processors, digital signal processors, or one or more
microprocessors and associated memories.
[0090] It is apparent from the above that the arrangements
described are applicable to the wireless communication
industry.
[0091] The foregoing describes only some embodiments of the present
invention, and modifications and/or changes can be made thereto
without departing from the scope and spirit of the invention, the
embodiments being illustrative and not restrictive.
* * * * *